In a heat exchanger, such as for heating and cooling of potable water or cooling of electrical transformers, a double-wall structure is desired between adjoining passages through which separate fluids are circulated, to prevent cross-contamination between a primary flow and a secondary flow of the heat exchanger.
In connection with plate type heat exchangers a double-wall structure has already been proposed in FR-A1 2454075 which discloses a plate heat exchanger wherein two identically corrugated plates are put together. A gap is formed between the two plates of the double-wall, which gap communicates with the atmosphere and through which a leakage of a fluid can escape to the atmosphere. Further, an interjacent wire netting can be used to increase the heat transfer between the plates. US 4 249 597 also discloses a plate heat exchanger having a double-wall structure. The heat exchanger comprises a plurality of identical plates assembled in pairs. Each plate has a series of protuberant channel portions looping back and forth between the short sides and along the long sides of the plate in an even number of parallel rows. Between said parallel rows there is a planar portion. The planar portions of each pair face each other and sealingly abut to provide a brazeable connection and seal therebetween. Two pairs of plates are then arrangeable to closely nest with each other by turning the two pairs 180.degree. with respect to each other. The channel portions at opposed sides of said pairs of plates nest alongside each other providing surface-to-surface contact between the channel portions. However, the planar portions at opposed sides of said pairs of plates are spaced from each other providing channels therebetween, through which any leakage can escape to the atmosphere.
The two above mentioned references only partly show a surface-to-surface contact between the plates of the double-wall structure and therefore the heat transfer between the plates has not proved to be efficient enough.